if this was the only reason for installing a cap, it would be enough. I have seldom seen an uncapped chimney over five years old that was not suffering from some kind of water damage.
Go over to your fireplace right now and look at the back firewall near the base. Take a screwdriver or coin and run it across the mortar between the bricks. Seem a little crumbly? Or maybe it's obvious just looking at it.
Rain puddles up on the smoke shelf, mixes with creosote in the chimney and turns into a highly corrosive acid which seeps down and attacks the mortar joints on the back wall of the fireplace. The mortar becomes weak, and presto, the bricks on the wall become loose. Besides that:
Rain can set off a bad smell in the chimney. This will happen in warm weather, especially if the chimney is dirty or has animal droppings on the smoke shelf.
Damper hard to move or frozen?
Rain speeds the rusting of metal parts of your fireplace.
A Cap Keeps Out Birds, Squirrels and Other Critters
Birds are fun to look at but they do have a few bad qualities when perched on your chimney. They chirp and flutter constantly, driving sane folks nuts, and crazy folks bananas. Their droppings accumulate in a huge vulgar mass on the smoke shelf causing a bad smell, and is a breeding ground for mites, and bacteria.
A squirrel can wreck your house faster than a dozen two-year-olds. Once they get inside the chimney, the only way out for them is through the damper.
Raccoons love to raise their young in the nice, dark, and warm smoke shelf, just a couple feet from your living room.
A Cap Inhibits Back-puffing
Back-puffing (fireplace smoke coming back into the house) can result from several factors. One of them is downdrafts.
A Cap Keeps Out Leaves
Leaves can choke a flue and set off chimney fire in a dirty flue. As you can see, there is probably no single investment that you can make for your chimney that will return so much to you. You have invested $5,000-$10,000 in your fireplace, why not protect that investment.
Which chimney cap is best for your chimney? Ask a professional, contact Mr. Sweeper now. He has the experience in your local area to help you make an informed decision.